Multilevel converters are generally known. They are used, inter alia, as reactive power compensators for non-linear loads. The load can be, for example, a three-phase electric arc furnace. However, other loads are also possible. Purely by way of example, with respect to the configuration and with respect to the basic mode of operation of a multilevel converter used as a reactive power compensator, reference is made to U.S. Pat. No. 6,075,350 A.
By means of the procedure known from U.S. Pat. No. 6,075,350 A, it is already possible to ensure that disruptive harmonics and reactive power components are filtered out. However, the procedure in U.S. Pat. No. 6,075,350 A is technically very complex. Furthermore, the known procedure cannot ensure that the desired active power is distributed uniformly among all the phases of the three-phase network.
The specialist paper “A Universal STATCOM with Delta-Connected Cascade Multilevel Inverter” by F. Z. Peng et al., 35th Annual IEEE Power Electronics Specialists Conference, Aachen, Germany, 2004 discloses performing both reactive power compensation and active power symmetrization in a multilevel converter.
A similar disclosure can be found in the specialist paper “Symmetry Compensation using a H-Bridge Multilevel STATCOM with Zero Sequence Injection” by R. E. Betz et al., Conference Records of the 42nd IEEE Industry Applications Conference, October 2006, pages 1721 to 1731.
A power converter for a motor, which is operated with an unbalanced AC power source and supplies the motor with current in a uniform fashion, is known from the specialist paper “Analysis of a Multi-Cell Converter under Unbalanced AC Source” by M. A. Perez et al., 36th Power Electronics Specialists Conference, Piscataway, N.J., USA, 2005.
US 2008/174183 A1 discloses compensating harmonics in the current and/or voltage of a three-phase current system.
WO 2005/029669 A2 discloses filtering harmonics and reactive currents as well as symmetrizing the active power in a three-phase current system by means of an active filter.
U.S. Pat. No. 5,648,894 A discloses compensating harmonics and load asymmetries in a three-phase current system.